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tv   Democracy Now  LINKTV  October 4, 2012 8:00am-9:00am PDT

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let's begin. here is a glass. what's in the glass, gang? - air. - yeah, the air. okay, what are we studying about today? atmospheric pressure in the air, right?
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what if we weren't studying about atmospheric pressure? isn't it common to say, "oh, there is nothing in the glass." then you take the glass and you say, there is nothing. and you can show a little ball, floats... look at the air in the glass gang. can you see evidence of the air in the glass? see that. there is a little ball, it's floating on the surface and the surface of the water goes down because something pushed the surface of the water down. begin with an a, end with a "r," try it. the air, and there is air in the glass. yeah, there is air in the glass gang. any air in this room? isn't it nice that there is air in the room. what if there is no air in the room? we'll not survive. you would be trouble. have you wondered about that like you're breathing and you really need the air-- in your nighttime, you're breathing all of a sudden you think, "wait a minute, if all the air molecules that are right here all of a sudden goes somewhere else and i go to take a breath and there is nothing there. honey, you're gonna go out, you're gonna check out. aren't you glad the air is all around--
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how come the air stays near where your mouth is? because it's being pushed by a pressure and that pressure is due to the weight of air above us. air has any weight? it turns out air has weight, a lot of weight if you talk about a lot of air, okay? what's it called here? one cubic meter of air, it's not like this, huh, it's like a lot like this. bam-bam-bam, one cubic meter of air, like a little small truckload gang, has one and a quarter kilograms, okay? that's about 2-3/4 pounds. so try this tonight when you go home. open up the fridge. now your fridge is about three-quarters of a cubic meter. ask the people at the house, "hey, the air inside there have any weight?" what are they going to say? yeah, it's got some weight. see that big grapefruit sitting there on the shelf, does that have any weight? yeah. say which weight is more, the grapefruit or the air?
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take a guess. you've got about 2 pounds of air in there. and if that grapefruit is less than a 2-pounder, then the weight of air in that refrigerator is greater than the weight of-- so greater than the weight of a dozen of eggs, dozen of eggs weigh less than the air in an ordinary-sized refrigerator. now you're not noticing that weight of air. let me ask you a similar question. does water have a weight? yeah, water kind of weighs a lot, doesn't it? if i handed you a great big bag of water and i say, "here, take this." and i put it in your lap, vrooom. and i ask you, "does water have any weight?" what are you gonna say? you are gonna say, "yeah, it's got weight." now let's suppose we go in a swimming pool and you're under the water. and under the water i take that baggy and i hand you that baggy of water and you grab it and i say to you, does the water have any weight? and you're gonna say, "if it does, i can't feel it." what does a fish know about the weight of water?
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nothing. fish knows nothing about water-- fish doesn't know what it's like not to have water, okay? we're the same way. we walk through the air all our lives, and we just take it for granted. we don't realize it's there, and we don't realize it's heavy. in your kid sister's room-- the air in your kid sister's room has more mass than your kid sister does, if she has an ordinary-sized bedroom. air is quite heavy, one and a quarter kilos for every cubic meter. got quite a few cubic meters, you've got more than a ton of air in an airplane to keep it pressurized. so air is kinda heavy, relatively heavy. and we don't notice the weight because why? we're in it. it's like the fish doesn't know the weight of water. now air has-- air has pressure too and air has pressure because of the weight. just look at the world. here is the whole world here, okay? i'm going to draw the top of the atmosphere, how high should i go?
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you know what gang, the top of the atmosphere is within the line that i drew. because the air only goes up about 30 kilometers, and this is over 6,000 kilometers. so, 30 parts in 6,000. gang, that's within the thickness of the truck, all right? so it turns out there is a very, very thin, thin vapor of air hugging the world and that's our atmosphere. it's not inexhaustible. take a pool ball and put it in a fridge, huh. let it cool. open up the fridge and say huh, breathe on it. you get a little mist on it, right? you know how thick that mist is? about the proportion of the thickness of our atmosphere. we've a very very thin vapor 30 kilometers, not very much, you know, compared to the world 6,000 kilometers radially. so it turns out that that thin vapor of air nevertheless has a weight and pushes down on us. it pushes down on us with a pressure. i'll show you an example of atmospheric pressure.
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see these cans? i put a little water in them, the water is boiling. and so, are there fewer molecules in there now than before or more? well, you can see 'em coming out, stuff must be going out. so it must be that the pressure inside there is now realized by fewer molecules moving faster. does that make sense? a few molecules moving very fast can exert as much pressure as a greater number of molecules moving slowly. so i'm reducing the number of molecules in there. what if i cool this very suddenly? shall we try it again? what happened to these cans, gang? let's try one more time. look at these cans. some people say, "oh, the cans are sucked in." cans ain't sucked in. what's happening to those cans? check your neighbor, what's happening to those cans? now when you try that tonight, gang,
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you're gonna want some explanation. a nice explanation will wait until we get up to heat and get up to condensation. when we get up to the idea that when steam condenses, okay, it loses volume. but for now, a first order explanation would just be that in here there is heated air. and heated air means not so many molecules. but the pressure inside the can is actually a little bit more than the pressure outside. can you see evidence of that? see the steam coming out. so the pressure is building up because they're moving fast, fast, fast, yeah? what i do is i remove it from the heat, put it over here and cool it. and when i cool it what happens? the pressure inside goes down. how about the pressure outside? some people say, "oh, the pressure outside must have all of a sudden increased." and you would say, "no, it didn't all of a sudden increase, it's what it is all the time." and that humongous pressure pushed the can in because there wasn't the same pressure inside to hold it out again. we're gonna come back to these cans again when we talk about condensation, water changing to vapor
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and vapor changing back to water, that--those ideas. but nevertheless, there is an enormous pressure due to the atmosphere and i think you can see that now. isn't it nice that you guys have air inside you? what happens all of a sudden someone takes the air outside of you? what are you-- what are you goi to look like? here you are, here gang. isn't it nice you've got the air inside you too, yeah, okay? okay. if i had a bamboo pole, a bamboo pole right here and it went up about 30 kilometers and that bamboo pole was one square centimeter in area. and i considered the mass of all the air that would fill up that bamboo pole, you know, what all of that mass would be? it would be one kilogram, one square centimeter up about 30 kilometers will be one kilogram of air. so that 1 kilogram pushes down with a weight.
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and it pushes down with a weight of 10 newtons. so it turns out the pressure due to the atmosphere here at sea level is 10 newtons for every square centimeter. and we're talking about si units, we talk about square meters. it's a little unwieldy, but it turns out it's about 10 to the fifth square centimeters and one square meter, and it turns out if you have one square meter, think of a great big sewer pipe about one square meter in cross-section. and that sewer pipe goes up 30 kilometers, you have about 10 to the fifth newtons pushing down and that's the atmospheric pressure, 10 to the fifth newtons per square meter and that's due to the weight of air above. that being the case, i've got a question that you can answer. if you climb the top of a mountain and then measure the atmospheric pressure, would it be less, the same or a little bit more?
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you'll be checking the neighbor. hey gang, let's suppose you had a pet lobster and the pet lobster's way down there in the deep brine there and the pet lobster says, "you know what? it's just too much water pressure." and someone says, "well, why don't you climb that ledge. "when you get closer to the surface, the water pressure will be a little bit less." will that be a true statement or a false statement? true. it would be a true statement. and where do we live in? we live in an ocean of air. so we've got a pressure acting down on us. so when we climb a mountain top, are we getting closer to the surface? now there isn't a sharp sharp surface with the air like there is with the water. it kind of peters up and peters out, doesn't it? in fact, you guys know if you go about 3.5 kilometers high, is it 3.5 kilometers high? it's in the textbook, i forget the figures. yeah, 5.6 kilometers high gang. you've got half the air underneath you. and you go up to what is it, 30 kilometers high, okay? that's about 19 miles, 30 kilometers high, 99% of all of the air is underneath you.
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so you went flying in an airplane and the airplane pilot says, "hey, we're at 40 kilometers high." honey, he has made a mistake. you've got nothing to ride on up there, right? the wings aren't going to ride in the air. you'll run out of air. the air doesn't go up that far. this is kind of a neat little deal here. this is called the unicorn experiment. little bottle, piece of paper... match... light the paper on fire, fire burning, i cut off the oxygen and when i do that... follow me gang, i'll make your crops grow. what's going on here, gang? oh gosh, well, this is my new style.
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i'll see you guys down the wave tonight, all right? [laughter] a little tattoo there. what's going on here, gang? ah-ah. the heated air cooled and when the heated air cooled what happened to the-- the pressure inside? it went down. how about the pressure outside. same. but why does it hold to my head? why? how many people say, "well, it has something to do with air, but there is probably no reason for that, huh." it's sort of like this. one of these devices here. you know what this is, this is a physics experiment, right? how come it's sticking, gang? it looks like we all at times gonna have-- gonna have one of these, gang. are you looking what's going on there, huh? why? how many say, "oh, there is probably no reason for that."
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isn't the air pressure pushing against the side there? isn't the air pressure inside pushing back out? but not as much. because i squashed some of the air out, and when i squashed thair out, it starts to come back, it reduces the pressure inside. so i have reduced pressure inside, but atmospheric pressure outside. so i have more pressure pushing here than in there and so it stays. isn't that neat? you could explain this to someone. everyone knows this will happen. someone said, "oh, it's because of suction." all right, okay, it's because of suction. now explain the suction. and we can do it, can't we? more pressure on one side than the other end, huh? yeah. so a toilet plunger makes a nice suction cup gang, but you don't have to have a toilet plunger because this is like a suction cup too. you know what this is? it's just a regular piece of rubber, a piece of floppy rubber and i've got a hook on it. i'm going to put it on the stool, i'm going to lift it up, i'm going to lift it up. would you lift it up, please?
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lift it up by the hook. yeah-h-h, all right, like a suction cup gang. what's happening there? when this pulls up, can you be explaining such a thing to your friend? yeah-h-h. glass filled with water, yeah? okay, where is my cards? take a card, uh-huh. okay, look at that. no problem, no problem. still no problem. -- somebody said, "wait a minute, "ain't that wate ain't that water pushing down with the pressure." yeah. "well, don't you know that pressure pushed the card away?" no-- and your neighbor says-- say, if there was a little air bubble on that, and it still holds? yeah, what if there is air in there, gang?
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does that help it to stay or not help it to stay? not helps it to stay. let's try it with the-- like air in that, okay. when it fills with water, yeah. now, some people would say, why doesn't the water pressure push the card away? and your neighbor said, "because the air pressure pushing up on the card." see, let's look at this on the board. there is water pressure pushing down like that, but there is an air pressure pushing here, air pressure pushing here, air pressure pushing here. i'm denoting pressure with these forces that would really be the force due to the air pressure, you kind of see that. but there is more up than down. because in here that water pressure is not pushing down as hard as 30 kilometers of air. thirty kilometers of air pushing down a lot harder on that card than the water. now i would have to make the water taller. some of you people know,
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how tall a water column would it have to be to push down just as hard as 30 kilometers of air. some of you people know because you've read the text. check the neighbor and see if your neighbor knows. how many meters high of water will push down just as hard as 30 kilometers of air? okay, gang. what's the answer? how tall a water column would you have to have to push down just as hard as the atmospheric pressure is pushing down? 10.3. 10.3 meters, that's right. 10.3 meters, that's about 30 feet, about 30 feet high. if you have a 30-foot high column or 10.3-meter column of water that's going to push down just as hard and that card will not stay there, it'll push away. or if i have some air pressure back here, if i have atmospheric pressure back here, that atmospheric pressure plus this will push that away. and once this starts to come down and reduces the pressure enough such that it won't-- sometimes you can get an air column and a liquid in there. that's where the card starts to pucker, air pressure reduces and you still have less pressure pushing down than you have here.
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then, of course, there is a little surface tension that will help you in there too. but it's kind of neat, how that works. now if you want to make an instrument to measure air pressure, you could have one of these 10-meter high things of water. but better than use water, it's better to use something more dense, so your height wouldn't have to be so high. and what's the densest liquid we know of, gang? mercury. mercury is 13.6 times as dense as water. that means the column of mercury would only have to be 1/13.6 as high. and 1/13.6 of 10.3 meters turns out to be 0.76 meters, turns out to be 76 centimeters, turns out to be 760 millimeters. and so when you're talking about atmospheric pressure in terms of millimeters of mercury that's what you're talking about.
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see because if you look at the-- if you have like-- all you have to do is take a dish of mercury and that mercury will glug right out there and when that mercury column... is 760 millimeters tall, it's about like this, huh, okay? when it's that tall, that's going to push down on the surface with just as much pressure as the atmosphere outside and so it'll stand right there. now let me ask you this commonsense question. what if the atmospheric pressure increases, what's going to happen to the mercury? it's gonna push it right up further, isn't it? it's going to push it up further until it's pushing down with the same pressure that the atmospheric pressure is exerting, isn't that true? and so what would happen if you took this barometer, this is a barometer, now right? let's suppose you put it in an elevator and took a ride to the top of a skyscraper. what's going to happen to the reading? it's going to do down. you guys ever feeling some of these skyscrapers going up,
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you can feel the reduced pressure, you really can and that barometer would pick it up. and so the barometer would fall the higher you got because there is less air up there, less air pressure. it kind of makes sense, doesn't it? what's above the column of mercury? question. what's above the column of mercury? begins with a z. - zip. - zip, nothing, a vacuum. maybe a little bit of mercury vapor, but in the sense nothing. there is no air up there. see, because you would make this thing by filling it up completely with air. i should have mentioned that. i mean, completely-- you would make this by filling it complete with mercury, dip it over and the mercury will go-go-go-go-go-go-glug out and what's on the top, nothing, a void. so that would be like a vacuum, maybe a little mercury vapor, but certainly not air. because how does the air get there? can you see that, gang? i mean, i could take this. suppose this was 10.3 meters tall, i bring up like that and all of a sudden it would start to drop. see--suppose 10.3 meters here,
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then there would be nothing there. so it'll be a void. maybe a little water vapor, yeah, but there would be nothing, wouldn't be like this air there, there is nothing oh, i am uh-huh. you see that, yeah. this is kind of nice, you've seen people do this. is this a big deal that i can pour water from one container to the other? how many people say, "wow, look at that." oh man, poured the water from one container, am i-- am i glad i came today, huh? that's no big deal. do you want to see something-- this kind of a big deal gang? here is what i want you to do tonight. take a glass of water, tip it over, take a glass of air and pour the air into the water and ain't that nice, huh-huh? hey, you like that? you're glad you came, huh? is physics fun? physics, yum-yum or yuk-yuk. it can be yuk-yuk, but it can be yum-yum too, yeah.
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do you ever wonder about how they build-- how they pour concrete down under the river to make the bridge? do you ever wonder about that? i mean, you see these bridges in the water, right? and down it's all con-- how do they get the concrete underneath there? they have workmen go down there, do you know what they do? they have a great big thing like this, okay? just little tiny guys like this, and they bring it down, down, down like that, right down to the bottom and they are the guys who are down there working. but you know what, as i keep pushing this down, down, down, it turns out the pressure gets squashed, not the pressure, the air gets squashed and it starts to get more and more compressed. this is not deep enough that you can really see the difference. but if i push it 10.3 meters down, then the water pressure would be just equal to the air pressure at the top, so you'll have twice the total pressure, 10.3 meters under
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and that's going to squash the air up to half size. and that brings us up to boyle's law. boyle's law named after a dude by the name of robert j. law, okay, we're learning these things, all right? and boyle's law just says that pressure multiplied by volume at any one point will equal pressure times volume at another point. so get twice the pressure, you'll get twice the volume. you'll have half the volume. yeah, what i told twice the pressure will be half the volume. very good, lee, sometimes i goof a rooney a little bit, right gang, but does that make sense gang? so you get twice-- you get twice the pressure that's going to squash that stuff up. so the volume will be half as much, huh? what if you push--so you've got three times the pressure, then the volume will get squashed up to how much? a third. how about--i don't know if you can do this. let me try, seven times the pressure,
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okay, okay, now i shouldn't say seven, now let's say five times the pressure, a fifth right? how about nine times-- how about--all right, here is for the a students, 7.9 times the pressure... got you, got you, got you, got you, got you, how many people say, "you ain't got me, honey." it's 1/7.9 of the volume, show hands? [laughter] front row, next time. last time we talked about a problem, gang. and the problem was where we had a balloon. and the balloon is weighted down, so it just barely floats in the water. if it's just barely floating,
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what's the density of that balloon and weight compared to the water? equal, isn't that all right? so i could say mass/volume, weight/volume. the idea, the concept is the same. your question was when you push that balloon underneath the water and then let go, what's the balloon going to do and all you guys have reasoned it out and you said, "hey gang, if it was a piece of wood you pushed it under, what would it do?" come back up. if it just is the density of the water it would stay there, but a slight bit less, it would come back up, right? how about a balloon, though? balloon is going to sink, stay where it is or come back up? it will sink. let's make a hypothesis that it does sink. let's think about this logically, okay? kind of scientifically. let's say it sinks. if it does sink, if it does,
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logic now, what would have to happen to the density? it would have to be more, is that right? it would be more, okay. how many ways can you make the density greater? increase its volume. how many know that it's two ways? and how many know it's two ways because the definition of density tells you, okay? so if you make the balloon heavier, will you make it more dense? yeah, so if you can think of any reason when you push it under that it becomes heavier, then it would become more dense? then the hypothesis that it sinks would be true? okay, can you guys think of any reason whey it would be a little heavier when you poke it under the water? if you can it sinks. any takers? you should get the volume-- decrease in the volume. you're talking about volume, i'm talking about weight. i only think about one idea at a time. i get friends that think of only one idea at a time period. and with that one idea, they'll take up arms and do everything, my god, they don't think about the other side, you know.
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but hey, let's--let's look at this, weight. does the weight increase, gang? doesn't that get closer to the surface-- center of the world? huh? yeah, but it's under the water. the water is gravitating it back up too, yeah. it turns out the weight doesn't change, at least not at the first order. so if the weight doesn't change, the density doesn't change, therefore it doesn't sink. so it doesn't sink. ain't that neat? so hypothesis number two is that it floats. shall we go onto flotation? what's wrong with my reasoning like most people in the world? what's wrong with my reasoning? i'm not considering the whole story. and the whole story is not only weight, but volume, ahh. so what would have to happen to the volume if this density is going to get more? how many say the volume would have to get bigger too? everything gets bigger. you may want to make some bigger-- everything, come on, come on, the volume gonna get smaller. and when you push it under, can you think of any reason why the volume should scrunch up smaller?
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water pressure. can you? depth, water pressure. but the pressure of the water is only going to to be a little bit more. oh that's more, that's more than what it is now. that's right and that's going to make the volume a little bit more, a little bit less. and if the volume is a little bit less the density is going to be a little bit more and if the density is a little bit more, it's going to --sink. so it sinks, yeah, yeah, yeah. did i get that one right, gang, huh? okay. isn't that true too of you when you're swimming? when you're swimming down down down, doesn't the density-- doesn't the pressure push you into? and don't you become more dense too? and don't a lot of people that find they can-- they can both float to the top a few feet under get way way down there, gang you sink, you know. when you snorkel, yeah. snorkels are about this long, yeah? you're snorkeling, right gang? you get it pointing up with water level right here, you can breathe that, all right. that's no problem. how about--you make it longer?
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how about you take a snorkel like this. so i think i'll go a little deeper. i'm not into scuba tanks, honey. i'm just going to take this and go way down. now, can you go that deep? what happens you're that deep, gang? you get a lot of water pressure pushing in on you. and what's that water pressure pushing in your lungs. and what's that water pressure going to do to the volume of your lungs, scrunch them up, right? okay. so the pressure is gonna be more in there, right? now you've got more pressure in your lungs than up there. which way is the air gonna go? someone will go-- you can't go-- --get to a point down about this--this far. i get to about here. i could go this deep. and when i was that deep down here, that deep, i go... i could just just barely breathe, any deeper, augh. can't do it. so that's what that's all about. that's why you have to have pumps at the surface or you have to bring a scuba tank with you.
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and with that scuba tank, that's regulated so the deeper you go, the more pressure it feeds into your lungs. got a little regulator, and it keeps the pressure in your lungs the same as the pressure of the water outside without your lungs having to be scrunched down, so they blow up. what's the first thing that scuba types will tell you if you're at the bottom and you've got a mouthful-- a lungful of air and you abandon things and you go to the top, what's the first thing they tell you not to do? hold your breath. what's the thing that people tend to do? oh, i gonna get all the way to surface, i better save all i got, right? so you're-- what's going to happen you gang? yeah. just as this balloon sinks deeper and deeper and gets smaller and smaller, if it's at the bottom, very very small and i stretch to come to the top, what's going to happen to the size? what if that's you holding your breath down there, what's going to happen to your size? call rupture city, honey, rupture city, you're going to be in trouble. so they tell you let your mouth open and let the air come out and it keeps coming out, out, out. where did all that air come, you've got compressed air down there.
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and you let your mouth-- let the air come out as you rise. so you don't all of a sudden blow up like this and really hurt yourself. you wanna see what an exam question would be on a balloon question like this, gang? here is an exam question in there. as this balloon starts to sink, will it sink all the way to the bottom? how many people say, "oh no, it'll get down "to a certain elevation like everything and kind of just stop, even rocks, man." you throw a rock overboard, it'll finally go down, the pressure will get so much it'll just kind of hang there. they all are scuba diver types, have you done that? they'll say, "watch out for those rocks, honey." rocks hanging in the air. when you throw a rock off, does it sink all the way to the bottom? and will that balloon sink all the way to the bottom? okay, now here is a question i got to ask you. as this balloon sinks deeper and deeper, the buoyant force that acts on the balloon will actually become more and more. no, no, no, no, no, no. the buoyant force that acts on the balloon
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will actually become less and less. no, no, no, no, no, no. the buoyant force that acts on the balloon will stay the same all the way down, yeah. check your neighbor. what's the answer, gang? what's going to happen to the buoyant force, as you sink deeper and deeper? sinks. what is it? how many say, the buoyant force will get more and more because you're getting deeper and deeper, more and more pressure, show hands? how many say, no it stays the same. one thing i learned, everything is the same. the cry of the ignorant, oh, everything is the same. how many say, no it'll get less and less? how many of you say, i am not sure, i am not going to stick up my hand making a darn fool of myself. [laughter] okay, we'll ask that question another time. hey, we talked about wood floating and you guys know why wood floats because the buoyant force acting up equals the weight acting down, isn't that true? in fact, isn't it true that anything that's submerged in any fluid will displace the weight of fluid equal to the vol-no-da-da-da- da-da. how does that go now? anything submerged in a fluid will be buoyed up by a force
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equal to the weight of the fluid. now we said fluid and not liquid, why? how about someone says, i can see why a fish doesn't sink. and i can see why a hunk of wood doesn't sink, but i don't see why that air-filled balloon doesn't sink. wait a minute, wait a minute, if you see one you've gotta see the other because the air-filled balloon is displacing air, yeah? and isn't there a buoyant force on the air-filled balloon? and if that buoyant force is more than the weight of the balloon, what will the balloon do? it'll go up. if that buoyant force is equal to the weight of the balloon, what will that balloon do? stay there. and if that buoyant force is less than the weight of the balloon, it will sink. so if i take a balloon and i blow it up with air and i let it go, it sinks. why does it sink? the balloons you see at the end of the string, at the circus, honey, those are filled with helium, okay?
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but why is it you blow a balloon up with air and it sinks. how about that air gang? is that air more dense or less dense than the air outside? it's a little bit more dense. do you know why? because the stretched rubber is tending to compress it. so it's slightly compressed. so it's more dense than outside. that means its volume of air will weigh more than the equal volume of-- this is going to sink, but they use helium. the helium is very very light, not as heavy, huh. each molecule is not as heavy. so it is not as dense. so the helium-filled balloon is simply lighter than the buoyant force and up it goes. but here is a point too. we get that buoyant force because of a difference in pressure. is that right? a difference in pressure, always more pressure on the bottom because it's deeper than the top. so i got a question. if someone says to you, "hey, i understand "the atmospheric pressure keeps getting less and less "the higher you go.
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"at the top of a mountain, there is less air pressure. "in cities like mile high, denver, in colorado, "there is less air pressure than in low places like at sea level." and they say, "i understand that." and they say, "but would there be more air pressure here compared to here?" is there more air pressure where my lower hand is compared to here, gang? it turns out if a helium-filled balloon this size will rise, then that is evidence that the pressure here really is more than the pressure here. see, archimedes' principle never told you why there was a buoyant force. it was a nice little way of telling how much buoyant force, but the reason you get a buoyant force when you are swimming and you're buoyed up with, remember, this part of your body is deeper than this part, or a balloon,
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this part is deeper in more pressure than this part and so up the balloon goes. it turns out if you move that air, if the air is in motion, then you get a nice effect. moving air doesn't push with as much pressure. if i have some slow-moving air traveling over the top of this table, here are some molecules-- hear that? i'll do it again. it's air pressure. now, i'll have the air moving faster. in which case was there more pressure? when air moves fast, the pressure it exerts is less. this little analogy here has a weakness, but it'll allow you to understand that a little bit. let me show you what i mean. i have here an air jet. and i am going to take a ball, and i am going to put the ball in the air jet and air is going to pass faster
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on one side of the ball than the other. now where the air is moving faster, what's going to happen to the pressure, gang? and where it is slower it's going to be-- more. and that means there is going to be a pressure difference and that might act on the ball and hold it up. follow me, i'll make your crops grow, okay? okay. now we've got one more i think in here, but a little one. you kind of get the idea, gang. now what's going on there?
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do you ever wonder why an airplane stays up-- an airplane? okay. an airplane wing is curved. the air moving across this part here is going faster than here. and you might think because it's going faster, it's gonna push more, but it doesn't, okay? it's like this. you have air traveling across the top of the wing. the air will move faster over here, over here, if it keeps up. you have this air here and it gets over here, it's gone a greater distance in the same time. it turns out the air will move faster over there. water flowing down in a brook, here is a top view of a brook and all of a sudden it gets narrow. when the water is coming down here, going through the narrow part, it'll speed up. or going through across these points here, it'll speed up. even if you take this off and let it go the water will speed up as it goes through this narrow part. air coming across here, suppose we have like a constriction here,
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the air would have to speed up when going through here, even if the constriction is not there. air will go faster over the curved part than this part and when it goes faster, it doesn't bump in as much as-- as when it's going slower. like if this is my wing and air is going over the top part and it goes like this and over the bottom, more pressure on the bottom. now it's not quite that simple because when i do this, i've got twice as much air going across there too. it's going twice as fast, it's got twice as much air. over here, i have got... is that right? to say, air is going twice as fast, to say it must have twice as much if you have the same density. so at least it's nice to think of going fast, not hitting so hard... going slower hitting harder that helps you to remember that slower moving air pushes with more pressure
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than faster moving air and that's what gives you a lift. now that's bernoulli's principle and birds fly that way too. birds have their wings curved, and the air moves a little faster over the top than the bottom. i've got a question for you. bernoulli was in the 1700s. do you know what birds did before the time of bernoulli? birds couldn't fly, they couldn't fly. birds were ground struck. they came walking along the ground, that's why they have legs. all birds have legs, okay? show me a bird with no legs, okay. and then along came bernoulli and v-r-o-o-o-m, flight and now birds can fly. so why? physics, gang, good for you.
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